Polytrimethylene terephthalate (hereunder abbreviated as “3GT”) fibers have long been known as revolutionary fibers having properties similar to nylon fibers including a soft feel and excellent elastic recoverability due to a low elastic modulus, and easy dyeability, as well as properties similar to polyethylene terephthalate (hereunder abbreviated as “PET”) fibers including wash-and-wear properties, dimensional stability and yellowing resistance. However, until very recently, no inexpensive production process for the starting material 1,3-propanediol had been established, and therefore industrial production of 3GT fibers has not been possible. An industrial production process for 1,3-propanediol was established in the late 1990s, leading to an immediate increase in 3GT fiber business.
From the standpoint of preservation of the environment and cost reduction, a new synthetic fiber business requires establishment of recycling techniques for the polymer waste, fiber waste or used fiber product waste created during the polymer or fiber production stages. In particular, it is essential to establish chemical recycling techniques whereby various types of recovered polymer waste are chemically decomposed for their recycling into useful raw materials.
For example, a chemical recycling technique for PET is disclosed in Japanese Unexamined Patent Publication HEI No. 6-72922 as a process whereby waste PET is hydrolyzed under hydrogen pressure in the presence of a hydrogenated catalyst to obtain terephthalic acid. Also, Japanese Unexamined Patent Publication SHO No. 53-63338 discloses a process whereby a polyester comprising an aromatic dicarboxylic acid and an aliphatic glycol having 2-6 carbon atoms is decomposed in the presence of a fatty acid such as acetic acid and an ester-exchange catalyst, and the aromatic dicarboxylic acid is recovered.
However, chemical recycling techniques for the 3GT fiber business has a very short history and has not yet been established.
3GT has a chemical structure similar to PET or polybutylene terephthalate, which may suggest that chemical recycling techniques for PET may be directly diverted. However, investigation by the present inventors has revealed that 3GT is much more susceptible to chemical and thermal degeneration than PET, and therefore when recovery techniques for PET are directly used, the recovery yield and quality of useful monomer is greatly reduced.
In WO97/49652 there is disclosed a process for monomer recovery from waste of polyester containing 2-70% of non-polymer components, and most of the description therein pertains to techniques for PET. Although this publication does describe the use of 3GT as an example of a polyester, the examples mention that methanol gas is blown into the 3GT at above 200° C. for a long period of 12 hours for depolymerization, and therefore when it is used for repolymerization the quality is greatly inferior to that of polymers obtained using virgin monomer.
Thus, since no chemical recycling technique has yet been established for 3GT, the expected future increase in 3GT business definitely implies the need for creation of such a technique.